1. Field of the Invention
The invention relates to a method and a structure of manufacturing an inductor in a monolithic circuit, and more particularly to a method and a structure of manufacturing an inductor with a high-quality factor and an air trench.
2. Description of the Related Art
The continuous miniaturization of integrated circuits (ICs) is a main trend in the semiconductor industry for the purpose of not only obtaining smaller sizes and lighter weights but also reducing manufacturing costs. Today, many digital circuits and analog circuits, such as complicated microprocessors and operational amplifiers, have been successfully mass produced into ICs by very large scale integrated (VLSI) technology. In general, the above-mentioned circuits include active devices, such as bipolar junction transistors (BJTs), field effect transistors (FETs) and diodes, and passive devices, such as resistors and capacitors.
However, miniaturization techniques have not been completely developed yet for certain circuits applied in specific areas, including, for example, radio frequency (RF) circuits, which are applied in communication equipment, such as cellular telephones (i.e., mobile telephones), cordless telephones, wireless modems and son on. Miniaturization of the RF circuits hinges on the ability to manufacture inductors with an appropriately high quality factor. Currently, the quality factor of inductors manufactured by semiconductor technology is less than 5, which does not meet desirable requirements. Although certain low-resistance metals, such as gold, can be used to increase the quality factor, it cannot be implemented by the current semiconductor technology.
It is well known that the quality factor represents the qualities of produced inductors. It can be estimated by the following formula:   Q  =      K    ⁢                  ω        ⁢                  xe2x80x83                ⁢        L                    R        s            
wherein xcexa9 is angle frequency, L is inductance, and RS is series resistance. Under an ideal condition, the quality factor Q of a non-loss inductor (that is, R=0) is approximately infinite. Even though it is impossible to manufacture the ideal inductor in the real world, an inductor with a high quality factor can be definitely obtained by decreasing the energy losses thereof.
Referring to FIG. 1, an equivalent circuit of a real inductor is shown. It can be considered that the real inductor consists of an ideal inductor L, a resistor Rs and a capacitor Cd, wherein the ideal inductor L and the resistor Rs are connected to each other in series and then are coupled to the capacitor Cd in parallel. Generally, the resistor Rs of a spiral metal line used for forming the real inductor is considered to be a main factor in reducing the quality factor thereof. One way to resolve this problem is to widen the metal line. However, this increases the area occupied by the metal line and the parasitic capacitance Cd that follows. It is obvious that the increased area is opposed to the miniaturization of the inductor. The parasitic capacitance decreases the self-resonance frequency of the inductor, which, as a result, limits the range of the operating frequency thereof. On the other hand, the quality factor Q is directly proportional to the angle frequency and is inversely proportional to the series resistor, so the metal line cannot be optionally widened.
In view of the above, an object of the invention is to provide a method and a structure of manufacturing an inductor with a high quality factor and an air trench in a monolithic circuit. The inductor manufactured by the invention has a lower series resistance and a lower parasitic capacitance. Therefore, the inductor of the invention has lower energy losses, a higher quality factor and a higher operating frequency.
To attain the above-stated object, an inductor in a monolithic circuit according to the invention has the following structure. A plurality of spiral metal lines formed over a substrate. A plurality of dielectric layers, each of which is formed between two adjacent spiral metal lines. A plurality of via plugs formed in the dielectric layers to connect two adjacent spiral metal lines to each other. A spiral air trench formed along the spacing of the spiral metal lines in the dielectric layers. In such a structure having a plurality of spiral metal lines stacked on each other with the via plugs therebetween, the series resistance thereof is greatly decreased without widening the inductor. Moreover, air contained in the spiral air trench with a lower dielectric constant can efficiently reduce the parasitic capacitance of the inductor. Hence, the inductor manufactured based on the structure has a higher quality factor.
A method of manufacturing an inductor according to the invention comprises the following steps. A plurality of spiral metal lines aligned with each other is formed over a substrate. A plurality of dielectric layers, each of which is located between two adjacent spiral metal lines, is formed over the substrate. A via plug is formed in each dielectric layer to connect two adjacent spiral metal lines. An upper dielectric layer is formed over the spiral metal lines. A spiral air trench is formed in the dielectric layers along the spacing of the spiral metal lines.